1. Field of the Invention
This invention relates to a platen for use in industrial machines, particularly presses such as injection molding machines, and particularly, but not exclusively, plastic injection-molding machines operating with a large clamp force/closure tonnage.
2. Background Information
Platens are essentially robustly constructed support structures that locate, support and align mold halves under applied clamp/closure tonnage. Each platen in a system must therefore be arranged to convey force effectively to the mold. In a molding process, as will be understood, it is important to maintain a high degree of parallelism between surfaces of a platen (and also between platens), since distortion in the platen correspondingly and usually induces distortion in a mold half (and particularly the mold face) located within the platen. Indeed, under applied clamp tonnage, platens should ideally be entirely resilient to physical distortions in their structure, thereby ensuring that a mold surface remains undistorted and hence substantially (and preferably entirely) flat. It is important to maintain, as far as possible, the flatness of the mold and platens.
In use, mold-bearing faces of co-operating platens are moved relative to each other to cause the formation of a mold through the aligned abutment of complementary mold halves. Under subsequently applied clamp tonnage, injection of melt can commence into a resultantly mold cavity defined by the two mold halves. More typically, and as will be understood, hydraulic actuators (typically dedicated pistons) are arranged to cause retraction of the tie bars (or the toggle clamp) by relative movement between a moving and stationary platen. Once in abutting and locked engagement, hydraulically actuated pistons generate a clamp force that is conveyed through the platens. Molten plastics material may then be injected (by an injection unit) into a mold cavity defined by the mold halves, thereby to form an article having a predefined shape. To enhance productivity, the mold halves are cooled by a water cooling system comprising a number of water carrying tubes, which system increases the rate at which the injected (e.g. molten plastics material) solidifies. The clamping force is then removed and the mold halves opened/separated to allow ejection or extraction of the molded article. The process can then be repeated.
Maintenance of flatness is important for many reasons. For example, by ensuring flatness, mold closure tonnage can be reduced; this saves energy and reduces potential wear between contacting elements. Approaching the issue of flatness (or, in fact, the lack of flatness) between platens and the mold halves from a reverse perspective, mold halves that are fractionally misaligned through bowing of their contact faces can suffer from part formation problems, usually related to mold “flash”. When a mold and platen arrangement is closed and clamped-up in a first phase of the molding operation an equilibrium is reached but the injection of plastics material into the mold halves can cause a separation line to open up because of a lack of uniform back-up for the mold. This causes mold flash to be formed.
There are several undesirable effects pertaining to the formation of flash in a molding process. Assuming that a resultant part is usable, an additional processing technique must be employed to remove the flash; this adds to time and cost in producing a product. Also, the removal process itself may not be entirely successful and some flash may therefore remain to spoil the finish of the article. Alternatively, flash may simply render the finished article unusable either as a consequence of the part being physically deformed or significantly underweight. Flash also generally increases wear in/of the mold to degrade considerably the service life of the mold, and can further result in more generally damage to the machine when the mold is either mechanically opened or cleaned. More specifically, after flash formation during an injection cycle, solidified material (which may not be evident from simply inspection of the mold) acts to bond the mold halves together, which bond is then only broken by sufficient applied force. Removal of the flash, from a region around the mold split line, in a manual cleaning process then subjects the mold to an increased risk of damage, while cleaning is nonetheless time consuming and so affects overall productivity.
In the injection molding industry, for example, clamp tonnage varies from several hundreds of tons of closure pressure to several thousands of tons of closure pressure. With increasing clamp tonnage, even platen structures made from the strongest materials, particularly steel, can undergo distortion, principally as a consequence of a bending moment being induced in the structure by a force path through the molding machine and mold. The closure or clamping force is typically applied by either a hydraulic piston or toggle-clamp structure, with a force path usually completed through a tie-bar arrangement.
Of course, platen and mold deflection problems can be overcome by increasing closure tonnage or producing a solid block of material. However, increasing applied tonnage can act to reduce the life-expectancy of the mold, whereas increasing the physical size and robustness of the platen results in higher energy requirements (and hence higher operating costs).
In contrast with the more common box-section platen design, U.S. Pat. Nos. 5,593,711 and 5,776,402 both discloses a platen for an injection molding machine, which platen has first and second generally planar walls with an intermediate support structure linking the walls. This platen is commonly known as a REFLEX® platen and is marketed under trademark REFLEX® by Husky Injection Molding Systems, also the assignee of the present invention. The walls within this REFLEX® platen are parallel with respect to each other. A first wall is arranged to support location of a mold half therein. A cone-shaped intermediate support structure of the REFLEX® platen design operates to redistribute forces acting in the corners of the rear wall of the platen towards the centre of the mold supporting face (i.e. the front wall), with the intermediate support structure providing controlled compression (through its spring-like configuration) that promotes flatness by reducing the effects of tensile forces (arising from a bending moment) across the rear wall. In addition, the REFLEX® platen reduces overall weight of the platen. The REFLEX® platen design therefore offers considerable improvements in maintaining flatness over earlier box-section designs, irrespective of whether such box-section designs are implemented in a toggle clamp or tie-bar system.
A REFLEX® platen is shown in prior art FIG. 1, with an enlarged corner view shown in FIG. 4. This platen, whilst including the cone-like intermediate support structure, also further includes intermediate structural support ribs which are arranged perpendicular to the first and second walls, i.e. the ribs run parallel to the direction of the applied force. The ribs act to increase the rigidity of the arrangement (particularly by reducing bending moments across the rear wall) and thereby to relay some relatively small portion of the total forces (generated during clamp up) between the front and back walls.
Referring particularly to FIG. 1 (but also the sectional view in FIG. 2), active forces are shown as arrows. Tensile forces are represented by arrow heads pointing towards each other, whereas compressive forces are represented by arrow heads pointing in opposite directions. As can be seen in FIG. 1, forces from the edges of the rear wall predominantly act through the cone (i.e. the intermediate support structure) which therefore supports/undergoes compression between the front and rear faces. Forces in the platen are derived from the applied force F acting through the mold, and a return (reaction) path (FR) acting from the point of tie-bar connection (at the back of a tie-bar support in each corner of the platen) and through the tie-bars themselves. In more detail, at clamp up and under applied tonnage, mold halves are pressed between the mold mounting face of the front wall of the platen and a further platen. The clamping force (F) causes compression in the walls of the cone of the intermediate support structure. These compression forces are resisted by the shape of the cone which, at the extreme pressures caused by the clamping, act in a spring-like manner. This spring-like operation is important in resisting forces which would otherwise result in a degree of non-flatness of the mold mounting face and the mold halves.
Perpendicular support ribs, located towards the centre of the two parallel walls, provide a secondary force path that runs from the point of tie-bar connection (on the rear wall) to the front face. The perpendicular support ribs are provided in pairs along each side of the platen, with the support ribs located towards a central area of the platen; this is best seen in relation to FIGS. 1 and 4. More specifically, the secondary force path is initially directed at ninety degrees (90°) to the plane of the rear wall before it runs through the perpendicular support ribs. The position of the support ribs results in the secondary force path producing a bending movement that reduces flatness between the rear and front faces, which reduction in flatness further results in bending being induced into the tie-bar by virtue of the tie bar being mechanically attached to the rear wall. Any bending in the tie-bar potentially causes metal fatigue and an increased risk of failure of the tie-bar, even though the REFLEX® platen design heretofore described acts to minimise such stresses.